President Barack Obama's pick for the next secretary of the US Army said Thursday he is worried about broad cuts to the service, which is set for a dramatic downsizing.

Eric Fanning, who was nominated to the Army's top civilian post in September and would be the first openly gay person to fill the position, told a long-delayed Senate confirmation hearing that budget cuts were reducing military preparedness at a time of growing international instability.

The Army is due to shrink to 450,000 active duty soldiers by 2018, down from a peak of 570,000 during the wars in Iraq and Afghanistan.

Later cuts could see troop numbers reduced further still, to 420,000.

"I do worry about the size of the Army today," Fanning said.

"Two years ago when we targeted 450 (thousand,) we didn't have ISIL, we didn't have Russia as provocative as it is," he added, using an acronym for the Islamic State group.

Though he was nominated months ago, Fanning's confirmation has been stalled by political squabbling.

Kansas Republican Senator Pat Roberts blocked his confirmation for reasons related to the closure of the Guantanamo Bay military prison.

Roberts wants guarantees that no inmate from the controversial facility would ever be transferred to a Kansas federal prison site under consideration in the event Guantanamo closes.

But lawmakers in the Senate Armed Services Committee on Thursday seemed set to vote in favor of Fanning's nomination, which the full Senate would also need to approve.

"You are about to be secretary of the Army, I think you are well qualified," Republican Senator Lindsey Graham said. "I look forward to voting for you."

Fanning has held an array of different posts in Congress and at the Pentagon over the past 25 years, including as an undersecretary of the Air Force, a deputy undersecretary in the Navy and he was chief of staff to Defense Secretary Ashton Carter.

Fanning said he supported women, who are now eligible to apply for any job in the US military including in commando units, to be required to register for the draft in case a national crisis sees the re-institution of mandatory conscription.

"If we are focused on equal opportunity, I think a part of that is equal responsibility," he said.